The present invention relates to a two-part adhesive composition, especially to a two-part adhesive composition containing a polyamine hardener, and a urethane resin composition comprising a modified polyurethane prepolymer having an epoxy group at each of its respective ends and a modified urethane compound having at least two epoxy groups.
In the aerospace industry, defense industry and semiconductor industry, an adhesive that is capable of hardening at room temperature and maintaining superior adhesive strength at cryogenic temperatures, such as -186.degree. C. or lower, is desirable. Common type or high-temperature-resistant type adhesives generally suffer a rapid deterioration of adhesive strength and embrittlement at low temperatures due to the development of internal stress. Thus, they are not satisfactory for use in the aforementioned applications.
According to "Selecting Structural Adhesives for Temperature Extremes," pp. 5-78, Materials Engineering (Appendix 1), amine cured two-part urethane adhesives, silicone adhesives and polyamide modified epoxy adhesives are known to be capable of maintaining a high adhesive strength at cryogenic temperatures. However, amine cured two-part urethane adhesives are not suitable for use in highly humid environments. The adhesive strength of these amine cured two-part urethane adhesives is very sensitive to the dew point of the atmosphere during its application to substrates. Polyamide modified epoxy adhesives deliver much less adhesive strength than that of amine cured two-part urethanes, and are further limited by having to be cured at high temperatures (&gt;150.degree. C.). Similarly, silicone adhesives generally deliver less adhesive strength than that of amine cured two-part urethanes at cryogenic temperatures; furthermore, their high temperature adhesive strength is not high either.
Aliphatic diamines are generally not used as hardeners for amine cured two-part urethane adhesives because they react too rapidly with the terminal ether cyanate groups of urethane prepolymers. Sandler, et al. (Applied Polymer Science, Vol. 9, pp. 3909-3916 (1965)) discloses a polyurethane adhesive composition that is suitable for use at cryogenic temperatures. The disclosed adhesive composition consists of an ether cyanate terminated urethane prepolymer made by reacting a polyetherpolyol with an excess amount of toluene diisocyanate (TDI). Then TDI or the condensation products of TDI and a polyol are reacted with a 3,3'-dichloro-4,4'-diaminodiphenylmethane (MOCA, Du Pont) hardener. However, the disclosed adhesive composition suffers from the fact that it must be cured at temperatures higher than 165.degree. C.